1. Field of the Invention
The present invention relates in general to a process for producing bone substitute materials, and particularly to porous materials capable of supporting or encouraging bone ingrowth into its pores. The present invention also relates to a process for preparing a rigid reticulated article, such as a bone substitute material.
2. Description of Related Art
Bone substitute materials are described in copending application Ser. No. 08/942,557 (now U.S. Pat. No. 6,136,029) and Ser. No. 08/944,006 (now U.S. Pat. No. 6,296,667). A variety of materials have been proposed for use as bone substitute materials, ranging from shaped porous metal objects suitable for defect filling around knee and hip joint replacements on the one hand to shaped ceramic materials on the other. Ceramic materials by and large have been formed through a sintering process in which a powder of a ceramic material such as zirconia is compressed to a desired shape in a mold and is then heated to sintering temperatures. The porosity of the resulting material is commonly quite low. Materials employing calcium phosphates (for example: fluorapatite, hydroxyapatite, and tricalcium phosphate) can also be sintered in this manner, the calcium phosphate having the capacity for acting as a substrate for bone growth (osteoconductivity).
It has been suggested to mix ceramic powders such as zirconia and hydroxyapatite, and fluorapatite and spinel, and then compress the mixture in a mold and either sinter or hot isostatically press to produce a somewhat porous ceramic of zirconia having pores at least partially filled with hydroxyapatite. Reference is made to Tamari et al., U.S. Pat. No. 4,957,509, and also Aksaci, D. et al., Porous Fluorapatite/spinel Osteoceramic for Bone Bridges, Ceramic Transactions, Vol. 48 p. 283 (1995). It has also been suggested to use ceramic articles having both high porosity and low porosity portions, and reference is made here to Hakamatsuka et al., U.S. Pat. No. 5,152,791, Johansson, U.S. Pat. No. 5,464,440 and Borom, U.S. Pat. No. 4,237,559. See also Klawitter et al. U.S. Pat. No. 4,000,525. The latter reference refers to the use of an Al2O3 slip that is foamed into a sponge, followed by firing.
By and large, metal or ceramic materials that have been proposed for bone substitutes have been of low porosity. The art contains examples of substantially dense metals and ceramics with a semi-porous surface which is filled or coated with a calcium phosphate based material. The resulting structure has a dense metal or ceramic core and a surface which is a composite of the core material and a calcium phosphate, or a surface which is essentially a calcium phosphate. The bone substitute materials of this type commonly are heavy and dense, and often are significantly stiffer in structure than bone. Reference here is made to U.S. Pat. No. 5,306,673 (Hermansson et al.), U.S. Pat. No. 4,599,085 (Riess et al.), U.S. Pat. No. 4,626,392 (Kondo et al.), and U.S. Pat. No. 4,957,509 (Tamari et al.).
In addition to bone substitute materials described above, there are other applications in which the chemical, thermal, or other properties of a ceramic, metal, or other material can best be used in a porous form. One form of rigid porous materials with utility is the reticulated foam. A foam material is one with a large degree of volumetric porosity. This porosity is ideally fully open and fully interconnected. A common method of manufacture of these types of materials is to coat the surfaces of a polymeric foam with a slip of ceramic or metal, and then burn out the foam and other organics. The ceramic or metal coating is then sintered to leave a rigid foam with a structure largely similar to the starting polymeric foam. There are numerous examples of cellular, rigid foams in the known art and processes for producing these materials. See, for example, U.S. Pat. Nos. 4,000,525 and 5,061,660. However, the known art for producing rigid foam materials suffers from occluded openings and thus is not able to achieve a substantially fully open and interconnected porosity.